The presence of polarization mode dispersion (PMD) can be a limiting factor in the design of optical transmission systems, particularly those providing long haul transmission of signal data streams of 10 Gb/sec or more over single mode fibers of the order of 100 kilometers in length. Although such fibers are nominally “single mode”, propagation is generally characterized by two orthogonally polarized HE11 modes for which slightly different group velocities exist in the presence of birefringence. Accordingly, for an arbitrary polarization of an optical signal at the input end of the fibre, the optical signal at the output end of the fibre will consist of both polarization modes separated by a certain amount of group delay. Cross-coupling of energy between the polarization modes, in the presence of this differential group delay (DGD), causes waveform distortion and consequent degradation of optical signal quality.
Optical transmission systems using data transmission rates of up to about 10 Gb/s are normally able to tolerate polarization mode dispersion on the order of 0.2 pico seconds per √km. Future optical transmission systems are expected to achieve data transmission rates of 40 Gb/s (or more), and thus are more likely to be limited by the effects of polarization mode dispersion. The effects of PMD in a high bandwidth optical link is discussed in “Temporal Dynamics of Error-Rate Degradation Induced by Polarization Mode Dispersion Fluctuation of a Field Fiber Link”, Henning Bulow et al., Proceedings of the 23rd European Conference on Optical Communications, IOOC-ECOC '97, Edinburgh, UK, Sep. 22-25, 1997. The impact of PMD in high bandwidth networks is expected to be particularly severe in systems which incorporate cross connected networks of fibers, in which an optical signal can follow any one of a number of possible routes utilizing different fibers (within the same or different cable), each with individual properties.
The amount of polarization mode dispersion varies from fiber to fiber, being dependent upon the amount of intrinsic birefringence associated with core asymmetry or frozen-in stress; extrinsic birefringence associated, for example, with cable induced stress, fiber bends or twists; and polarization coupling between optical elements within a link. As a result of these factors, PMD tends to be a statistical vector quantity which varies with both wavelength and time. For a detailed description of PMD, see “Long-Term Measurement of PMD and Polarization Drift in Installed Fibers”, Magnus Karlsson et al., Journal of Lightwave Technology, Vol. 18, No. 7, (July, 2000). Various methods are known for measuring PMD in an optical transmission system, such as, for example, as disclosed in U.S. Pat. No. 5,949,560 (Roberts et al.).
In the prior art, there are three general categories of techniques used for PMD compensation, namely: all-optical, all electrical and hybrid. Both the electrical and hybrid PMD compensation methods involve the optical-electrical conversion of the optical signal traffic, and thus suffer increasing performance degradation as data bit-rates exceed about 10 Gb/s. For this reason, all-optical PMD compensation, in which the optical signal traffic remains in the optical domain, is the preferred choice for high bandwidth optical communications.
U.S. Pat. No. 6,240,748 (Henderson et al.) entitled “Frequency and Amplitude Modulated Spins for PMD Reduction” teaches reduction of PMD in a single mode fiber by spinning the fiber during the drawing process in accordance with a spin function having sufficient harmonic content to achieve low levels of PDM. This approach attempts to manipulate the intrinsic birefringence of the fiber to reduce the overall differential group delay, and thus PMD, experienced by light propagating through the fiber. However, this technique is inherently is incapable of addressing the effects of extrinsic birefringence.
U.S. Pat. No. 5,473,457 (Ono), entitled “Method and Apparatus for Compensating Dispersion of Polarization” teaches an all-optical PMD compensation system in which an optical signal received through an optical fibre is passed through a (high birefringence) polarization maintaining fibre. A polarization controller arranged between the optical fibre and the polarization maintaining fibre is used to rotate the PSP of the received optical signal, so that the polarization maintaining fiber imposes a PMD that is equal and opposite to that of the optical fiber. However, the success of this technique relies of the assumption that the optical fibre has a known PMD which remains constant. As pointed out above, PMD is typically time and wavelength-dependent, so the method of U.S. Pat. No. 5,473,457 (Ono) can, at best, compensate for the mean PMD of the optical fibre.
In the article entitled “Component for Second-Order Compensation of Polarization Mode Dispersion” by J Patscher and R Eckhardt (July 1997), a cascade of polarization controllers and short polarization maintaining fibres are used to compensate the PMD of a long single-mode fibre. This arrangement enables compensation of a greater range of PMD than that of the system of Ono, but otherwise suffers from many of the same disadvantages.
International Patent Publication No. WO/01/86840 (Bandemer et al.) published on Nov. 15, 2001 teaches an all-optical PMD compensation system in which a cascade of polarization controllers and short birefringent elements are used to compensate the PMD of a long single-mode fibre. An emulator is used to analyze the optical signal received through the optical fiber, and model the PMD of the optical fibre as precisely as possible. PMD model is then used to control the cascade to produce an equal and opposite PMD. This arrangement enables compensation of a greater range of PMD than that of the systems of Ono and Patscher et al. The emulator also provides real-time modeling of PMD, and thus addresses the issue of time-variance of PMD.
U.S. Pat. No. 6,104,515 (Cao), entitled “Method And Apparatus For Providing High-Order Polarization Mode Dispersion Compensation Using Temporal Imaging” teaches an all-optical PMD compensation system in which an optical phase modulator is controlled by a sinusoidal clock signal that is frequency-locked with the data signal, and phase-delayed by 90° relative to the data signal. As a result, for each bit passing though the optical phase modulator, a leading edge of the bit is retarded, and the trailing edge of bit advanced. Since this phase modulation effect is substantially independent of polarization mode, the net effect is that the fast polarization mode is retarded and the slow polarization mode advanced, thereby correcting DGD. This arrangement suffers the disadvantage that it relies on a precise phase relationship between the data signal and the sinusoidal clock signal controlling the phase modulator. Such precision can only be attained when there is only one data signal within the fibre. Accordingly, the system of U.S. Pat. No. 6,104,515 (Cao) must necessarily be duplicated for each channel within a WDM optical transmission system. This greatly increases costs, and introduces “dead bands” between channels, within which no PMD compensation is possible.
U.S. Pat. No. 6,130,766 (Cao), entitled “Polarization Mode Dispersion Compensation Via An Automatic Tracking Of A Principle State Of Polarization” teaches an all-optical PMD compensation system in which the polarization modes of a received optical signal are rotated to align with a polarization beam splitter. As a result, one of the polarization modes (ideally the polarization mode having the highest optical power) passes through the beam splitter and continues to the system output via an optical amplifier. The other polarization mode is used by the control unit. Rather than compensate PMD, this system eliminates it by physically removing one of the polarization modes entirely. This arrangement suffers the obvious disadvantage that up to half of the total optical power of the received optical signal is lost, if the received optical signal is unpolarized. If the received optical signal is polarized, then the entire optical signal can be lost, depending on whether or not the polarization direction of the received optical signal is corresponds with the mode used by the control unit.
Each of the above-described references suffer the further disadvantage that PMD is typically non-linear across the range of wavelengths used in WDM optical transmission systems. The system of U.S. Pat. No. 6,104,515 (Cao) necessarily requires per/channel demuxing of the optical signal within a fibre, because only one optical signal can compensated. In principle, the systems described in each of the other references can provide broadband PMD control. However, in this case, each wavelength will be subject to the same optical processing, resulting approximately constant PMD compensation across the wavelengths of interest. Where PMD is varies, per-channel demuxing of the optical signal is necessary, so that each channel can be treated independently. This increases the cost of the system, and introduces dead-bands between the channels, in which no PMD compensation is possible. Furthermore, per-channel PMD compensation necessarily ties the system to the channel plan (i.e. number and wavelength separation between channels) of the optical transmission system. Consequently, any changes in the channel plan necessitates expensive modification (or outright replacement) of installed PMD compensation equipment.
Accordingly, a technique for implementing broadband control of polarization mode dispersion remains highly desirable.